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Extraction of Source Parameters for French Seismicity Based on a Radiative Transfer Approach: Importance for Attenuation and Site Corrections

An accurate magnitude estimation is necessary to evaluate properly the seismic hazard. Unfortunately, magnitudes of small earthquakes are subject to large uncertainties due to high-frequency propagation effects, which are generally not accurately considered. To address this issue, we developed a method to separate source, attenuation and site parameters from the elastic radiative transfer modeling of the full energy envelopes of seismograms. Our separation method is based on a 2-step inversion procedure. First, for each source-station pair, we retrieve optimal frequency-dependent attenuation parameters fitting the observed energy envelopes in the 0.375-24Hz band. In a second step, we separate the source spectra and the site amplification relative to (1) the network of stations and (2) a selection of stations showing small amplification (characterized by approximatively flat H/V ratios). From the source spectra, we estimate the moment magnitude Mw, the corner frequency fc and the stress-drop ∆σ. We applied the inversion procedure to more than 16000 waveforms recorded by EPOS-FR stations from 662 earthquakes of magnitudes Mw ranging from 2.0 to 5.0. We show that the choice of the reference in site amplification affects in a critical manner source parameters (Mw, fc and ∆σ), highlighting the necessity of a careful selection of the source-site separation criteria minimizing site amplification bias on the estimated seismic source. More specifically, we observe an increase of ∆σ with Mw if the site amplification is determined relatively to the whole network of stations. However, if the site amplification is relative to selected stations with approximatively flat H/V ratios, ∆σ does not scale with Mw. The resulting magnitudes are compared to existing Mw catalogues in France (Si-Hex, FMHEX-22, Buscetti et al. SSA meeting). The role of attenuation maps is explored according to the stabilization of the site and source terms separation. In the future, we intend to automate our method and apply it routinely to smaller earthquakes for which traditional methods are not readily applicable due to the complexity of waveforms.

Session: Understanding and Quantifying the Variability in Earthquake Source Parameter Measurements - I

Type: Oral

Room: Tubughnenq’ 3

Date: 5/3/2024

Presentation Time: 08:15 AM (local time)

Presenting Author: Grégoire Heller

Student Presenter: No

Additional Authors